This invention relates to an active matrix-type liquid crystal display device and a method of making the same. More particularly, it relates to a novel structure of wiring, such as signal lines and scanning lines, and pixel electrodes formed on an array substrate and a manufacturing process for it.
An active matrix-type liquid crystal display device is provided with an array substrate, a counter substrate disposed opposite to the array substrate, alignment layers formed on the array and counter substrates, respectively, and a liquid crystal composition held between the substrates through the alignment layers. The array substrate includes an isolation substrate, such as a glass substrate, a plurality of signal and scanning lines made of low electric resistance materials, such as aluminum, in matrices, thin film transistors (TFTs) provided at cross points of the signal and scanning lines, respectively, and indium tin oxide (ITO) pixel electrodes connected to the TFTs. An isolation layer is interposed between the scanning lines and the pixel electrodes to define storage capacitors. The surface of the array substrate is covered with the alignment layer to align the liquid crystal composition.
The active matrix-type liquid crystal device of this sort is generally manufactured in the following processes. In the first step a predetermined composition layer, e.g., an ITO layer is formed on a glass substrate by sputtering or chemical vapor deposition (CVD). In the second step a photoresist is coated on the ITO layer. The photoresist is optically exposed through a photomask with a predetermined pattern in the third step. The photoresist is selectively removed from the ITO layer by developing in order to transfer the pattern thereto in the fourth step. The layers under the photoresist are subject to etching, e.g., wet or dry etching through the remaining photoresist. As a result, the mask pattern is formed on the layers in the fifth step. Finally, the remaining photoresist already used as a mask is removed from the ITO layer so that the pattern defining steps are completed.
A plurality of composition layers with a predetermined pattern are formed on an array substrate through repeating such layer forming processes a prescribed number of times. In the processes for manufacturing an array substrate set forth above, a plurality of masks are necessarily used for separate exposure steps to make a predetermined pattern on a same layer. In other words, since one sheet of photomask cannot cover a liquid crystal display device pattern in the case of a larger array substrate in accordance with a larger display area of a liquid crystal display device, a display area is divided into a plurality of optical exposure regions and optical exposure is performed for each of the optical exposure regions in patterning each layer.
When an optical exposure is performed for patterning a plurality of exposure regions divided in a display area, one after the other, on an array substrate as described above, the patterning accuracy depends largely on positioning errors of the optical exposure device used for the patterning processes. In short, after a first wiring pattern made of a first layer has been formed on an array substrate as a reference pattern, a second wiring pattern made of a second layer is formed. The second pattern may have relative positioning deviations with respect to the reference pattern in each of the divided exposure regions due to the positioning error of the exposure device.
When pixel electrodes made of an ITO layer as reference patterns are combined with the second wiring (signal line) pattern of an aluminum material, the exposure regions differ in distances defined between the pixel electrodes and the signal lines.
The wiring patterns formed in different process steps may cause relative positioning deviations in each of the exposure regions. Since electric force lines are distributed between the first and adjacent second wiring patterns formed on the isolation layer, a parasitic capacitance is defined between them in response to the density of electric force lines. In the above case, a parasitic capacitance is defined between the signal line and the pixel electrode.
Where the distance between the pixel electrode and the signal line is different from one of the exposure regions to another, they differ in the density of electric force lines and the capacitance defined between them from each other. As a result, effective voltages applied to the liquid crystal layer differ in each exposure region so that the brightness of the exposure regions is different from each other and boundaries between the regions appear on the display area. This leads to a problem of a poor quality display which is referred to as the unevenness of combined images.
One object of the present invention is to provide a liquid crystal display device which hardly causes the unevenness of combined images and a method of manufacturing the same.
An active matrix-type liquid crystal display device of this invention includes an array substrate, a counter substrate provided opposite to the array substrate, and a liquid crystal layer held between the array and counter substrates. The array substrate is provided with switching elements, such as thin film transistors, a plurality of electrically conductive lines connected to the switching elements, and a plurality of pixel electrodes connected to the switching elements.
Either component of the conductive lines or the pixel electrodes is made of first and second layers. Importantly, the other component not selected from them and the first layer are formed by a same patterning process using a same mask. With this structure, the electric coupling between the conductive lines and the pixel electrodes is substantially determined by relative positions between the first layer and the one not selected. Thus, in the event of a position shift with respect to the second layer, it does not affect the coupling critically. The first layers may be wider in width than the second layers while the second layers may be formed within the width of the first layer. Alternatively, the first layer may be formed to cover or wrap the edge of the second layer so that the mutual position can be kept constant between the conductive lines and the pixel electrodes.
More particularly, the conductive lines consist of signal and scanning lines. They are disposed to cross each other on the array substrate. The signal lines are made of first and second conductive layers. The first conductive layers and the pixel electrodes are formed by a patterning process using a same mask.
Alternatively, the pixel electrodes are made of first and second conductive layers. The first conductive layers and the signal lines are formed by a patterning process using a same mask.
A method of manufacturing an active matrix-type liquid crystal display device of the present invention includes the steps of making an array substrate, a counter substrate provided opposite to said array substrate, a liquid crystal layer held between said array and counter substrates, switching elements on said array substrate, a plurality of electrically conductive lines on said array substrate; and a plurality of pixel electrodes on said array substrate.
The conductive lines and the pixel electrodes are connected to the switching elements. One component selected from the components consisting of the conductive lines and the pixel electrodes is made of first and second conductive layers. The other component not selected from the components and the first layer are formed by a patterning process using a same mask. With this process, the electric coupling between the conductive lines and the pixel electrodes is substantially determined by relative positions between the first layer and the one not selected. Thus, in the event that a position shift of the second layer takes place, it does not critically affect the coupling. The first layers may be wider in width than the second layers while the second layers may be formed within the width of the first layer. Alternatively, the first layer may be formed to cover or wrap the edge of the second layer so that the mutual position can be kept constant between the conductive lines and the pixel electrodes.
The first layer is wider in width than the second layer. The second layer is formed within the width of the first layer.
Particularly, the method of manufacturing an active matrix-type liquid crystal display device set forth above is characterized in the steps of making thin film transistors on a array substrate, a plurality of electrically conductive signal lines on the array substrate, a plurality of electrically conductive scanning lines on the array substrate, and a plurality of pixel electrodes on the array substrate.
Further, the signal and scanning lines and the pixel electrodes are connected to the thin film transistors. The signal and scanning lines are disposed to cross each other. The signal lines are made of first and second layers. The first layers and the pixel electrodes are formed by a patterning process using a same mask.
Alternatively, a method of manufacturing an active matrix-type liquid crystal display device includes the steps of making the pixel electrodes made of first and second conductive layers, the first layers and the signal lines formed by a same patterning process using a same mask.
The manufacturing method of the present invention is also suitable for manufacturing a large display area on a glass sheet which is divided into a plurality of regions. Separate exposure processes are applied to them, respectively. Since the first layer of either component selected from the conductive lines and the pixel electrodes and another component not selected therefrom are formed by a patterning process using the same mask as set forth above, mask position shifts do not affect the coupling critically in the case of the separate exposure processes.
According to an active-matrix type liquid crystal display device and a method of manufacturing the same of the present invention, where each layer is formed on an array substrate by separating it into a plurality of exposure regions and by application of optical exposure thereto, the distance between pixel electrodes and wiring is kept substantially constant in each region so that there is no substantial phenomenon in which parasitic capacitance varies from one region to another. The present invention is capable of suppressing unevenness of combined images caused by different brightness of each display region corresponding to its exposure region.
The above-stated and other objects and technical features of the present invention will become apparent from the following description when taken with the accompanying drawings. It will be understood, however, that the drawings are for purposes of illustration and are not to be construed as defining the scope of the invention, reference being had for the latter purpose to the claims appended hereto.